Electronic devices come in many sizes and shapes. One type of device can be formed by mounting electronic components onto a substrate, such as a flexible polyester sheet. Some substrates can be quite small (i.e., credit card size or less), such that the resultant device formed is also small. There is generally an emphasis on decreasing overall device dimensions while increasing the overall performance and/or capabilities of a device. Classes of devices finding increasing use are active and passive radio frequency communication devices capable of tracking an object.
An example electronic device, and problems associated therewith which motivated the invention, are described with reference to FIGS. 1 and 2. An electronic device 10 includes a flexible polyester substrate 12 having conductive printed thick film ink traces 14 printed or otherwise formed thereon in a desired pattern. A thin profile battery 16 is effectively bonded to substrate 12 by cured conductive adhesive interconnections 17 and 20 which conductively connect portions of battery 16 to different portions of conductive traces 14. Thin profile batteries comprise batteries that have thickness dimensions which are less than a maximum linear dimension of their anode or cathode. One type of thin profile battery is a button-type battery. Such batteries, because of their compact size, permit electronic devices to be built which are very small or compact.
An encapsulant mass 18, for example insulative epoxy, is received over and adheres to thin profile battery 16 and substrate 12. A typical thickness for flexible substrate 12 is 5 mils, with an example thickness for printed thick film 14 being between 1 and 1.5 mils. An example thickness for encapsulant mass 18 is 90 mils.
At certain elevated temperatures occurring either during test, operation or storage, material inside of thin profile battery 16 can be caused to outgas into the surrounding encapsulant mass 18. The outgassing material can migrate through encapsulant mass 18 around or along cracks, crevasses and grain boundaries to a point where it reaches polyester film 12. The soft or more flexible nature of substrate 12 typically compared to the encapsulant mass 18 can cause the substrate and encapsulant mass to displace from one another the result of the expanding gas, particularly below battery 16. Accordingly, the gas can cause bowing or bubbling of the polyester film outwardly away from the encapsulant. Under such circumstances, the bowing or bubbling as a minimum creates an undesired appearing product. Even worse as shown in FIG. 2, the bubbling can cause rupturing of one or more conductive bonds between the printed thick film and battery as the printed thick film also pulls away from the encapsulant mass with the polyester film.
It would be desirable to develop structures which minimize or reduce problems associated with material which outgases from a gasifiable component received within an encapsulant mass.